1. Field of the Invention
The present invention generally relates to ultrasonic medical imaging apparatus and, in particular, to such apparatus which employ polyvinylidene fluoride piezoelectric transducers.
2. Statement of the Prior Art
The significance of ultrasonic medical imaging as a safe, non-invasive diagnostic tool has increased dramatically over the past ten years due largely to the availability of affordable, high-speed image processing computers. Generally, electrical signals are generated which are used to excite a piezoelectric transducer. The transducer transmits ultrasonic energy into a patient and then receives the echos thereof from the different tissue surfaces located within the patient's body. The received echos generate electrical signals in the transducer which are then converted to digital signals and processed to form a meaningful image of the orientation of the reflective surfaces.
Heretofore, the ultrasonic transducers used have primarily been ceramic piezoelectric materials. Limits have been encountered in the use of such material, however, as it does not readily lend itself to the formation of transducers having specially designed shapes.
In an attempt to avoid these shortcomings, other materials have been experimented with including polyvinylidene fluorides (PVF.sub.2). Materials of this nature may be readily formed into different shapes. Further, PVF.sub.2 has a low lateral coupling coefficient which allows for the construction of transducer arrays by photolithographic techniques. They also have a good ability to withstand shock. These materials enjoy superior bandwidth properties for transmission, which is an advantage affording better resolution at greater depth. Another advantage is that they have low acoustic impedances relative to ceramics, which obviates the need for special impedance matching of the transducer to the patient's body. One material which has been used in non-ultrasonic areas is a vinylidene fluoride/trifluoroethylene copolymer (VF.sub.2 /C.sub.2 F.sub.3 H) which has shown increased sensitivity over other PVF.sub.2 materials.
Unfortunately, the limitations of PVF.sub.2 materials are significant and have thus far seriously hampered use in the medical imaging area. They suffer from very low electro-acoustic conversion efficiency and very high dielectric losses. The inefficiency limits the peak power which may be used during pulse transmission, and the dielectric losses reduce the dynamic range of the material in the reception mode. The peak power limits during pulse transmission hinder resolution at depth. Further, the improved sensitivity is still not comparable to a wide variety of piezoelectric ceramics.